The process of embryo implantation is associated with uterine stromal cell decidualization, an event that is critical to the success of pregnancy. The decidualization process is characterized by stromal cell growth and transformation into decidual cells, and formation of stromal cell polyploidy which occurs due to endoreduplication without cell division. The process of cellular division is tightly regulated in the cell cycle at two particular checkpoints, G1-S and G2-M. Several cell cyclins (A, B, D and E type cyclins) are known to play key roles with regard to cellular growth in a phase specific manner during these transitions. In this regard, the PI has recently demonstrated that the upregulation of cyclin D3 is tightly associated with decidualization of the uterus. It is well accepted that the dynamic control of the cell cycle is executed by an interplay of cyclins, cyclin-dependent kinases (cdks) and cdk inhibitors (CKIs). The objectives of this revised application are to focus on the cell cycle regulators that participate in the onset and progression of decidualization aspects of the implantation process. The hypothesis is that cell cycle regulators are expressed in the uterus in a spatiotemporal manner and their coordinated interactions play key roles in these processes. The specific aims are: 1) To characterize the spatiotemporal expression of cyclins (A, B, D2 and E), cdks (cdk1, cdk2, cdk4 and cdk6) and CKIs (p27 and p21) in the periimplantation (days 1-8) uterus of wild-type mice; 2) To study the role of cell cycle molecules in decidualization and the effects of the EGF family of growth factors in this process; 3) To study the regulation and functional status of cdks, respective cyclins and CKIs in the uterus during implantation and decidualization in cyclin D2, D3 or p27 null mice; and 4) To assess the status of cyclins (A, B, D2, D3 and E), cdks (cdk1, cdk2, cdk4 and cdk6) and CKIs (p27 and p21) in the uteri of Hoxa-10 null mice. The investigators will characterize mRNA expression by Northern and in situ hybridization and protein expression by immunohistochemistry, Western blotting and immunoprecipitation. The functional activity of cyclin-cdk-CKI associated complexes will be determined by in vitro phosphorylation activity. The results obtained from these studies will provide important insights regarding the potential roles of key cell cycle regulators in uterine biology during implantation and decidualization. Since these processes are analogous to pseudomalignancy, the results should also provide basic information relevant to cancer biology.